Communication systems are known to support wireless and wire lined communications between wireless and/or wire lined communication devices. Each type of communication system is constructed, and hence operates, in accordance with one or more communication standards. For instance, wireless communication systems may operate in accordance with one or more standards including, but not limited to, IEEE 802.11, Bluetooth, advanced mobile phone services (AMPS), digital AMPS, global system for mobile communications (GSM), code division multiple access (CDMA), and/or variations thereof.
Depending on the type of wireless communication system, a wireless communication device, such as a cellular telephone, two-way radio, personal digital assistant (PDA), personal computer (PC), laptop computer, home entertainment equipment, et cetera communicates directly or indirectly with other wireless communication devices. To participate in wireless communications, each wireless communication device includes a built-in radio transceiver (i.e., receiver and transmitter) or is coupled to an associated radio transceiver (e.g., a station for in-home and/or in-building wireless communication networks, RF modem, etc.).
As is well known, the receiver is coupled to the antenna and includes a low noise amplifier, one or more intermediate frequency stages, a filtering stage, and a data recovery stage. The low noise amplifier receives inbound RF signals via the antenna and amplifies them. The one or more intermediate frequency stages mix the amplified RF signals with one or more local oscillations to convert the amplified RF signals into baseband signals or intermediate frequency (IF) signals. The filtering stage filters the baseband signals or the IF signals to attenuate unwanted out of band signals to produce filtered signals. The data recovery stage recovers raw data from the filtered signals in accordance with the particular wireless communication standard.
As is also known, the transmitter includes a data modulation stage, one or more intermediate frequency stages, and a power amplifier. The data modulation stage converts raw data into baseband signals in accordance with a particular wireless communication standard. The baseband digital signals are converted to analog baseband signals or intermediate (IF) signals. The transmitter filtering stage filters the baseband or the IF signals to remove images caused by the data digital signal processing and the digital to analog conversion process.
The transmitter filtering stage further processes the baseband analog or intermediate analog signals in accordance with the particular communication standard. The one or more intermediate frequency stages mix the baseband signals with one or more local oscillations to produce RF signals. The power amplifier amplifies the RF signals prior to transmission via an antenna.
To aid the reception and transmission analog signal processing various types of analog filters can be fully integrated on chip. Such filters can be resistor-capacitor (RC) filters (passive or active), switch capacitor filters and transconductor-capacitor (Gm-C) filters. An analog RC filter includes resistor-capacitor combinations to set the pole frequency of the filter. It may also include transconductance stages, commonly called gm stages, in various configurations to set the filter gain.
FIG. 2 is a block level diagram of analog filter 200. It comprises a transconductance stage 240, a resistor-capacitor R1-C1 combination and a transimpedance stage 390. Analog filter 200 is configured as a current mode filter where its input is current Iin and its output is current Iout. Transconductance stage 240 connects to a first terminal of resistor 260. A second terminal of resistor 260 connects to a first terminal of capacitor C1. A second terminal of capacitor C1 connects to ground. The second terminal of resistor 260 connects to transimpedance stage 390. The overall filter transfer function is defined as Iout/Iin.
The 3 dB cutoff frequency, therefore the filter bandwidth, for such a typical filter configuration can be approximately expressed as:
                              F                                    -              3                        ⁢                                                  ⁢            dB                          ≈                  1                                                    (                                                      R                    ⁢                                                                                  ⁢                    1                                    +                                      1                                          g                      m                                                                      )                            ·              C                        ⁢                                                  ⁢            1                                              Eq        .                                  ⁢                  (          1          )                    The gain of the filter may be changed by changing the size of the transconductance stage 240.
Modern communication protocols require that both the receiver and the transmitter filtering include gain control. In typical filters employing a transconductance stage, if the gain of the filter is changed by changing the transconductance, the filter pole locations may change resulting in filter bandwidth variations over gain control. However, bandwidth variation in a filter may lead to significant performance degradation in both receive and transmit signal paths of the wireless device. In the receive path, variations in the bandwidth of the receiver's baseband analog filter leads to performance degradation in static sensitivity, sensitivity in the presence of interferers, receiver IP3, and anti-aliasing performance. Alternately, in the transmit path, variations in the transmitter's baseband filter bandwidth leads to performance degradation in the transmitter's EVM (Error Vector Magnitude), ACLR (Adjacent Channel Leakage Ratio), and static/transient power mask performance.
Prior art has shown various implementations for calibrating the filter bandwidth and gain but these implementations tend to require significant calibration time. In mobile communication devices, particularly in transmitters, the gain of the analog filters can change very rapidly with stringent requirements on how much time is allowed between gain steps. Doing a sequential adjustment of the gain of the filter gain, first, and then of the bandwidth of the filter could violate the transmitter timing requirements. Hence, implementing on-chip filters to meet these stringent timing requirements is particularly difficult for multimode-multiprotocol communication systems. Thus, a need exists for on-chip filter structure and operational methodology for meeting these requirements.